1. Field of the Invention
The present invention is generally related to multi-layer composite cookware and, more particularly, to a ceramic coating that is selectively applied to protect the exposed edges of an aluminum layer in the composite against the corrosive effects of strong caustic cleaning agents.
2. Description of Related Art
The use of multi-ply composite materials for the manufacture of cooking and baking vessels has been known for approximately the last fifty years. These composite materials often involve highly wear-resistant, corrosion-resistant and chemical attack-resistant materials such as titanium or stainless steel and good heat energy conductors such as aluminum and/or copper. Typically, these materials are bonded together in slab or sheet form using various techniques involving heat, atmospheric control and/or mechanical deformation in the form of hot rolling or pressing. The resulting multi-layer composite may take the form of a flat composite blank which may then be draw formed into a cooking vessel or bonded as a flat composite arrangement on the bottom of a piece of already formed cookware.
The inert nature of stainless steel cooking surfaces combined within the heat buffering effect of the high heat conductor layer make bonded metal cooking and baking utensils superior in performance to single metal utensils, both from a cooking performance standpoint and also in clean-up and long service life. Although copper is considered the most efficient conductor of heat energy of the commercially available metals, aluminum has been the metal of choice for the conductor in most commercial composite cookware and bakeware utensils. This is because aluminum is lighter in weight and is more easily bonded to stainless steel than copper.
The use of interior layers of aluminum in composite cookware and bakeware is not without its drawbacks, however. Exposed aluminum at the edge of a multi-layered composite can be attacked and eroded by strong caustic cleaning agents. The use of such strong caustics is particularly prevalent in Europe and other locales where water conservation is prevalent. Typically, in such places automatic dishwashers use less water and stronger/caustic cleaning agents which accelerate corrosion exposure problems. The presence of this erosion leads to exposed sharp edges of non-eroded stainless steel which may cause a cutting injury to the user. This erosion also weakens the edge of the cooking vessel, leaving it subject to bending. This could be critical in the case of certain vessels, such as a pressure cooker, for example. Solutions to this chemical erosion problem have been developed. One approach is to carefully machine out (cut away) the aluminum from the edge of the pan or vessel and roll the stainless steel over the aluminum layer, thus protecting the aluminum from corrosive caustic solutions. This machining process requires great care and accuracy. The machining is not easily accomplished and is especially difficult to carry out on non-round shapes such as oval, square or rectangular vessels, which naturally cannot be milled and rolled on a rotating lathe.
U.S. Pat. No. 4,646,935 to Ulam, which is incorporated by reference herein, proposes the use of a stainless steel ring fitted around the peripheral edge of an induction cooking utensil to deter corrosion of a carbon steel layer present in the composite.
My invention provides a method to protect the exposed aluminum edge layer of composite cookware against corrosion and erosion. This is accomplished with a form of anodizing known as micro arc oxidation to treat the exposed aluminum edge. This process is commercially practiced by companies such as Mofra Tech and Micro Plasmic Corporation. The coating is applied to the aluminum by an electrochemical dip process and when in place is extremely hard, chemically resistant and has excellent adhesion to the substrate. The dip process allows for the protection of non-round exposed edges which heretofore could not be easily rolled over in the prior art method of protecting the edge by rolling.
Also disclosed is a method of protecting the stainless surfaces of the treated pot or pan from the erosive nature of the electrolyte used in the coating process.